1. Field of the Invention
This invention is directed to systems, devices, and methods for the treatment of sludge, and preferably, to systems, devices, and methods for manufacturing fertilizers, more preferably a bioorganic-augmented high nitrogen-containing inorganic ammonium fertilizer utilizing primarily organic sludges including municipal dewatered sludges or biosolids, concentrated acids and ammonia. The invention is also directed to products produced by processes of the invention.
2. Description of the Background
The disposal of sludges discharged from municipal wastewater treatment plants is a serious and growing problem. In 1990, the United States Environmental Protection Agency indicated that a family of four discharged 300 to 400 gallons of wastewater per day and in 2000 this number has almost doubled. From this wastewater, publicly owned treatment works generate approximately 7.7 million dry metric tons of sludge (or “biosolids” as these municipal sludges are now called) annually or about 64 dry pounds of sludge for every individual in the United States.
The definitions of “sewage sludge” and “sludge” and “biosolids” under Title 40 of the Code of Federal Regulations, Part 257.2, hereby incorporated by reference, is as follows:                “Sewage sludge means solid, semi-solid, or liquid residue generated during the treatment of domestic sewage in a treatment works. Sewage sludge includes, but is not limited to, domestic septage; scum or solid removed in primary, secondary or advanced wastewater treatment processes; and a material derived from sewage sludge. Sewage sludge does not include ash generated during the firing of sewage sludge in a sewage sludge incinerator or grit and screenings generated during preliminary treatment of domestic sewage in a treatment works. Sludge means solid, semi-solid or liquid waste generated from municipal, commercial, or industrial wastewater treatment plant, water supply treatment plant, or air pollution control facility or any other such waste having similar characteristics and effect.”        
For the purposes of the present application, the term sludge also encompasses municipal dewatered biosolids, domestic septage, heat-dried biosolids, pharmaceutical fermentation wastes, microbial digests of organic products such as food stuffs, food byproducts, animal manures, digested animal manures, organic sludges comprised primarily of microorganisms, and any combination thereof.
There are several types of sludges that can be produced from sewage and/or wastewater treatment. These include primary sludge, waste activated sludge, pasteurized sludge, heat-treated sludge, and aerobically or anaerobically digested sludge, and combinations thereof. These sludges may be from municipal and/or industrial sources. Thus, sludges can comprise macromolecules including proteins. Sludges can comprise personal pharmaceutical compounds, antibiotics, hormones, hormone-like molecules, other biologically active compounds, and macromolecules including proteins. Thus, an urgent need exists for an effective, efficient, and economical process for treating sludges.
Commonly, but inadequately, sludges are merely dewatered to the best extent possible by chemical and mechanical means. The water content of sewage sludges is still very high, and none of the undesirable compounds listed above are neutralized. Typical sludges coming out of a gravity clarifier may have a dry solids content of 2% or less. After anaerobic digestion, the solids content can be about 10%. Cationic water-soluble polymers have been found useful for causing further separation between the solids and the water that is chemically and physically bound. Filtration or centrifugation of cationic polymer treated sludge typically yields a paste-like sludge cake containing a range of solids, commonly from 18% to 30% solids.
Drying of sewage sludge (to greater than 90% solids) has been practiced for many years in both the United States and Europe. Sludge drying in the United States prior to about 1965 was undertaken to reduce transportation costs and in pursuit of various disposal options. In some plants, the sludge was dried in powder form and the fine particles were consumed in the combustion chamber of an incinerator or boiler. In the late 1960's two municipalities, Houston and Milwaukee, began to market a pelletized or granulated dried sludge for use as a soil amendment and/or fertilizer. Several more plants for manufacture of dried pelletized sludge were built in the 1980's and 1990's; especially after ocean dumping of sludge by coastal cities was eliminated. Drying and conversion to a heat-dried biosolids pellet fertilizer was the best option for these metropolitan areas where landfills and land for disposal were limited and even in 2007 it remains a very viable technology. However, the investment required for a sludge drying facility is very large resulting in municipal costs greater than $300 per dry ton of biosolids, and in many cases greater than $500 per dry ton of biosolids.
The most common type of sludge dried and pelletized is anaerobically-digested municipal sewage. Anaerobic digestion, as the name suggests, involves treatment by facultative bacteria under anaerobic conditions to decompose the organic matter in the sludge. After a prescribed time and temperature, a sludge relatively free of putrifiable organic matter is obtained. Unfortunately, some pathogens remain in such biosolids, and the USEPA has classed such treated biosolids as Class B implying that they are of a lower standard than the “Class A” treated biosolids. Because Class B biosolids contain pathogen indicators—and therefore potential pathogens, they are restricted in the manner by which they can be applied to animal and human crops. In contrast, Class A biosolids, e.g., heat-dried biosolids pellets, as well as the product of the present invention, are not restricted under current USEPA standards as fertilizer for animal or human crop usage.
If pathogens (e.g. Salmonella sp. Bacteria, enteric viruses, and viable helminth ova) are below detectable levels, the biosolids meet the Class A designation. The Part 503 rule (Title 40 of the Code of Federal Regulations, Part 503, incorporated herein by reference] lists six alternatives for treating biosolids so they can be classified in Class A with respect to pathogens. Alternative 1 requires biosolids to be subjected to one of four time-temperature regimes. Alternative 2 requires that biosolids meet pH, temperature and air-drying requirements. Alternative 3 requires that when biosolids are treated in other processes, it must be demonstrated that the process can reduce enteric viruses and viable helminth ova, and operating conditions used in the demonstration after pathogen reduction demonstration is completed must be maintained. Alternative 4 requires that when biosolids are treated in unknown processes, biosolids be tested for pathogens at the time the biosolids are used or disposed or, in certain situations, prepared for use or disposal. Alternative 5 requires that biosolids be treated in one of the Processes to Further Reduce Pathogens. Alternative 6 requires that biosolids be treated in a process equivalent to one of the Processes to Further Reduce Pathogens, as determined by the permitting authority.
Class A pathogen biosolids must also possess a density of fecal coliform of less than 1,000 most probable numbers (MPN) per gram total solids (dry-weight basis) or a density of Salmonella sp. Bacteria of less than 3 MPN per 4 grams of total solids (dry-weight basis). Either of these two requirements must be met at one of the following times: when the biosolids are used or disposed; when the biosolids are prepared for sale or give-away in a bag or other container for land application; or when the biosolids or derived materials are prepared to meet the requirements for Exceptional Quality biosolids.
All biosolids applied to the land must meet the ceiling concentration for pollutants, comprising 10 heavy metal pollutants: arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc. If a limit for any one of these is exceeded, the biosolids cannot be applied to the land without the incorporation of significant restrictions. Exceptional Quality (EQ) is a term used by the USEPA Guide to Part 503 Rule 7 to characterize biosolids that meet low-pollutant and Class A pathogen reduction (virtual absence of pathogens) limits and that have a reduced level of degradable compounds that attract vectors. It is the intent of this invention that all biosolids meet or exceed the USEPA's Exceptional Quality (“EQ”) standard for metal levels.
Pathogen reduction must take place before or at the same time as vector attraction reduction, except when the pH adjustment, percent solids vector attraction, injection, or incorporation options are met. Finally, vector attraction reduction must be met when biosolids are applied to land. Most commonly, this will be achieved by drying the biosolids product to a level of greater than 90% solids.
Sludge that is merely dried, as with heat-dried pellets, even if dried to greater than 90% solids, has several disadvantages for agricultural use. It has low fertilization value, typically having nitrogen content of only about 2-5%. Freight and application costs per unit of nitrogen are high. The heat-dried biosolids often has a disagreeable odor, particularly when moist. Also, dried pellets have low density and hardness and when blended with other commercial fertilizer materials, the pellets may segregate, and disintegrate and may not spread on the field uniformly with other more dense ingredients. Some bacterial action may continue when the material becomes moist, and under storage conditions, the material's temperature may rise to the point of autoignition. Hence, except for special markets that value its organic content for soil amendment or filler in blended fertilizer, there is relatively poor demand for the heat-dried biosolids product. In most cases municipalities must pay freight charges, or may offer other incentives for commercial growers to use the material. However, this is frequently still more economical than alternative disposal schemes.
The market value for agricultural fertilizers is principally based on their nitrogen content. A need exists for a practical, safe and economic method for increasing the nitrogen content of biosolids to a level approaching that of commercial mineral fertilizers, i.e., 10-20%. If such a biosolids fertilizer could be manufactured then overall value of the biosolids product and demand for the product would increase. Moreover, a properly manufactured biosolids fertilizer will have an advantage in that much of its nitrogen will be of the slow release type. This is very desirable since it provides nitrogen to the plant all through its growing cycle. Traditional inorganic manufactured slow release nitrogen fertilizers have a price many times that of ordinary mineral nitrogen fertilizers. Under the scenario of high nitrogen biosolids-containing fertilizer production from their biosolids, municipalities would enjoy public and regulatory support for their biosolids disposition program. Such a program would ensure the regular removal of their dewatered or dried biosolids, by, for example, recycling biosolids into a high nitrogen fertilizer which then can be sold directly into the mature national fertilizer distribution industry, thereby eliminating one of the major problems traditionally associated with biosolids treatment programs.
Prior attempts have been made to reach some of these objectives. U.S. Pat. Nos. 3,942,970, 3,655,395, 3,939,280, 4,304,588, and 4,519,831 describe processes for converting sewage sludge to fertilizer. In each of these processes a urea-formaldehyde condensation product is formed in situ with the sludge. Thus, the processes require the handling of formaldehyde, a highly toxic lachrymator and cancer suspect agent.
Other processes require costly process equipment and/or special conditions not readily incorporated in existing sewage treatment facilities (See: Japanese Patent No. 58032638; French Patent No. 2,757,504).
A simple method for increasing the nitrogen in sludge would be to blend commercial nitrogen fertilizer materials to the wet sludge prior to drying and pelletizing. There are only a few high-nitrogen fertilizer materials that are economical for use in agriculture. Examples are: ammonia (82 wt. % N), urea (46 wt. % N), ammonium nitrate (35 wt. % N). Ammonia has high volatility and is subject to strict regulation of discharges to the atmosphere. Urea is a solid that adsorbs moisture quite readily and makes the sludge more difficult to dry. Urea is also highly susceptible to breakdown to ammonia by the microbes and enzymes in biosolids if they are not properly prepared, resulting in nitrogen loss and an odor problem. Ammonium nitrate is a strong oxidizer and can result in a potential explosion problem which has all but eliminated this fertilizer from the commercial market after 2000. All of these fertilizers have high nitrogen content, but are less than ideal for combining with biosolids absent special processing.
Other references, such as European Patent No. 0143392 B1, Japanese Patent No. 9110570 A2, and “Granulation of Compost From Sewage Sludge. V. Reduction of Ammonia Emission From Drying Process”, Hokkaidoritsu Kogyo Shikenjo Hokoku, 287, 85-89 (1988)) fail to disclose the use of acids with ammonium sulfate additions and do not discuss the issue of corrosion of steel process equipment under acid conditions.
Over the past thirty years alkaline stabilization of biosolids has been a standard and successful method of making biosolids into beneficially useful materials that can be used principally as soil-conditioning materials. Because these alkaline-stabilized biosolids products have high calcium carbonate equivalencies, they have been produced and marketed as AG-lime materials, usually as a replacement for calcium carbonate in farm soil management strategies. Because of this usage, the value of these materials has been restricted to only a few dollars per ton of product; therefore, because of transportation costs especially since they usually contain significant water concentrations—often up to 50%—they are economically and geographically restricted to areas close to the source of their treatment.
Thus, there is a long standing need for practical means of increasing the economic value of sewage sludge through increasing its nitrogen content, and increasing its ability to be spread as well as a need to treat these materials such that they are converted into commodity fertilizers with physical and chemical and nutrient properties such that they can command significant value in the national and international commodity fertilizer marketplace. A series of U.S. Pat. Nos. 5,984,992; 6,159,263; 6,758,879 and 7,128,880 have described a method of production of high nitrogen organically enhanced ammonium sulfate fertilizers made with biosolids utilizing a pipe-cross reactor as originated by the Tennessee Valley Authority. The pipe, tee and pipe-cross reactor are defined by the IFDC in the Fertilizer Manual (1998), p 440 as:                “the pipe reactor consists basically of a length of corrosion-resistant pipe (about 5-15 m long) to which phosphoric acid, ammonia and often water are simultaneously added to one end through a piping configuration resembling a tee, thus the name “tee reactor.” . . . . The tee reactor was modified by TVA to also accept an additional flow of sulfuric acid through another pipe inlet located opposite the phosphoric acid inlet, giving the unit a “cross” configuration and thus the name “pipe-cross reactor”.Both the IFDC Fertilizer Manual (1998) and the Fertilizer Technical Data Book (2000) refer to the pipe-cross reactors. It was thought that pipe cross reactors delivered a more concentrated mix to the granulator shaping device and more efficiently evaporated undesired water from the fertilizer mix than other devices, but these references demonstrate a long-felt need for improvement, indicating that one of the shortcomings of the pipe cross reactor is scale formation inside the pipe which can result in clogging.        
The methodologies taught by this group of patents (U.S. Pat. Nos. 5,984,992; 6,159,263; 6,758,879 and 7,128,880) are plagued by problems related to the blockage of these narrow relative to their length reaction “pipe-like” reactor configurations during operation and related to the difficulty of control of the reaction temperature and pressure and retention time of the mix within such pipe-cross reactors. These pipe-cross reactors are narrow in contrast to their length, e.g., up to 6 to 8 inches in diameter and often 15 feet in length or longer. The plant practicing the manufacture of organically-enhanced ammonium sulfate fertilizers often had to shut down and disassemble the pipe-cross reactor either due to blockage from biosolids buildup or from destructive over heating in such reactors such that the commonly used “Teflon” coating on the interior-reaction side of the reactor was melted and ruined. Further, the use of the pipe-cross reactor has the distinct disadvantage of having very short reactor retention times (usually less than 20 seconds) which is an advantage in the manufacture of traditional fertilizers like ammonium sulfate but is a disadvantage when coupled to the simultaneous process of biosolids. Such short processing time increases the probability of untreated or non-homogenous mixing as the three material inputs pass through this reactor. Also limiting is the lack of control over the atmospheric pressure within such pipe-cross reactors since these reactors have open ended discharges usually directly into a granulator.
U.S. Pat. No. 4,743,287 issued in 1988 by Robinson described a method to use two reaction vessels in sequence to incorporate organic sludges into nitrogen fertilizers of low or medium nitrogen concentration (a range of 4 wt. % N to a maximum of nitrogen concentration of 10 wt. %). Robinson uses his first reaction vessel to achieve very low pH values of the mixture (pH 0.2 to 1.5) to achieve hydrolysis of some molecules present and to prepare the mix for reaction in a second reaction vessel. Robinson does teach that a single reactor can be used but only in a batch configuration and not in a continuous flow manufacturing method. In all cases, Robinson teaches that the acid and ammonia may be injected in either order but must be injected in sequence. This patent importantly describes the reaction vessels capable of achieving high pressures (30 PSI) with relatively long retention times as compared to the pipe-cross reactors. However, Robinson fails to meet the need for a novel and practical continuous flow method of manufacturing high nitrogen (greater than 8% wt. % N) and biosolids-containing fertilizer products under the advantages of defined temperatures, pressures and reaction retention times.